1. Field of the Invention
The present invention relates to a power supply apparatus.
2. Description of the Related Art
In small information terminals such as a mobile phone and a PDA (Personal Digital Assistance) of recent years, there are devices that require a voltage higher than the output voltage of the battery such as a light emitting diode (hereinafter also referred to as a LED) used as a back light of a liquid crystal, for example. In these small information terminals, a Li ion battery is often used, and the output voltage thereof is typically about 3.5 V, and is about 4.2 V in the fully charged state. However, the LED requires a voltage higher than the battery voltage as the driving voltage thereof. In this manner, when a voltage higher than the battery voltage is required, the battery voltage is boosted with use of a power supply apparatus of booster type using a charge pump circuit or the like, thereby to obtain a voltage needed for driving a load circuit such as a LED.
In driving the LED with such a power supply apparatus, the control of the luminescence brightness thereof is stabilized by connecting a constant current circuit on a path for driving the LED and keeping the electric current flowing through the LED to be constant (See Patent Document 1).
In order to achieve stable operation of the constant current circuit connected to the cathode terminal of the LED, the transistor constituting the constant current circuit need to operate in a constant current region. Here, the constant current region of a transistor refers to an active region in the case of a bipolar transistor, and refers to a saturation region in the case of a field effect transistor (hereinafter referred to as FET). The transistors constituting the constant current circuit is disposed in series between the cathode terminal of the LED and the grounded terminal. In order that this transistor may operate in the constant current region, the cathode terminal of the LED must be kept at a predetermined voltage or higher. Hereinafter, the voltage at which the constant current circuit can operate stably will be simply referred to as a stable operation voltage.
Here, in a power supply apparatus for driving the LED, the case of using a charge pump circuit capable of switching the boosting ratio is considered (See Patent Document 2). When the battery voltage that is input into the charge pump circuit lowers, the output voltage of the charge pump circuit, namely, the voltage at the anode terminal of the LED, also lowers. In accordance therewith, the voltage at the cathode terminal of the LED, which has been lowered by the forward voltage Vf from the voltage at the anode terminal of the LED, also lowers, so that the constant current circuit cannot be operated stably. Therefore, in this case, the constant current circuit can be stably operated by monitoring the voltage at the cathode terminal of the LED and switching the boosting ratio of the charge pump circuit so that the voltage at the cathode terminal of the LED may not become lower than the predetermined stable operation voltage.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-22929    [Patent Document 2] Japanese Patent Application Laid-Open No. H6-78527
In the case of switching the boosting ratio of the charge pump circuit by monitoring the voltage at the cathode terminal of the LED as described above, a threshold voltage corresponding to the stable operation voltage of the constant current circuit must be set, and the voltage at the cathode terminal must be controlled to be higher than this threshold voltage.
However, since the element characteristics of the transistors and the resistors constituting the constant current circuit fluctuate due to variation of the semiconductor manufacturing processes and the temperature, the stable operation voltage of the constant current circuit also fluctuates in accordance therewith. For this reason, the threshold voltage must be set to be higher in consideration of the process variations and the temperature characteristics. For example, when the stable operation voltage of the constant current circuit fluctuates within a range of ±0.1 V at the maximum with the designed value of 0.3 V at the center, the voltage of 0.4 V or higher is set as the threshold voltage so as to ensure the margin.
Here, in the case in which this threshold voltage is set to be 0.4 V, when the stable operation voltage of the constant current circuit becomes 0.2 V due to the process variations, the voltage of the cathode terminal, which should be stabilized inherently at 0.2 V or higher, is stabilized to be 0.4 V or higher. In other words, though the boosting ratio should inherently be raised when the voltage of the cathode terminal becomes 0.2 V or lower, the boosting ratio must nevertheless be raised at the time the voltage of the cathode terminal becomes 0.4 V or lower.
Since the efficiency of the charge pump circuit becomes deteriorated according as the boosting ratio becomes higher, the efficiency of the whole circuit will be deteriorated when a margin is set at the threshold voltage for achieving stable operation of the constant current circuit as described above.